Solid oxide fuel cells (“SOFC”) and other solid state electrochemical cells rely for their operation on the separation of a reduction-oxidation reaction into its two half-reactions that occur in physically isolated regions of a device, with a component called the electrolyte used to conduct ions between these regions. A simplified diagram depicting the structure is presented in FIG. 1 (Prior Art). Following the flows from top to bottom in the diagram, oxygen (or another oxygen-containing gas such as air) and electrical current diffuse and flow down through a porous cathode current collector 16 (“CCC”) to a porous cathode electrocatalyst 25 and then to a three phase boundary 3 (“TPB”) at the interface between the cathode 25 and the electrolyte 10. At the TPB the oxygen molecules are converted onto oxide ions (O2−) which then enter a dense electrolyte 10 and flow to an anode TPB boundary 5, where they (in the case of an SOFC) oxidize the gaseous fuel molecules and release to the anode 12 the electrons that they originally received at the TPB 3 of the cathode.
Solid oxide electrochemical cells require specialized materials in order to function. The electrolyte must be composed of a material that is gas-impermeable, has adequately low electron conductivity, and adequately high oxide ion conductivity. The details of a particular cell design determine the adequacy of these conductivities. Materials that provide generally acceptable properties for the electrolyte include Yttrium-doped Zirconium Oxide (“YSZ”), Scandium-doped Zirconium Oxide (“SSZ”), Scandium-Cerium-doped Zirconia (“SCSZ”), aliovalent-cation (a cation with different oxidation states than the host cation) doped Ceria and doped Lanthanum Gallate. Gadolinium-doped Ceria (“GDC”) and Samarium-doped Ceria (“SDC”) will provide adequate properties in an oxidizing atmosphere, however the electron conductivity increases to unacceptable levels in the reducing atmosphere of the anode side of the electrolyte at above 700° C. Doped Ceria is, therefore, still useful as an oxide ion conductor in the oxidizing atmosphere of the cathode, and can be used as a cathode or anode component or an electrolyte for low-temperature operation.
Another class of material that is required for solid oxide electrochemical cell operation is an electrocatalyst, which is a material that has an adequately high electron conductivity as well as surface activity for the adsorption and catalytic dissociation of oxygen molecules into atoms and reduction of the atoms to oxide ions. Such materials include platinum, silver, and Lanthanum-doped Strontium Manganate (LSM).
There exist materials which provide both useful levels of electron conduction as well as oxide ion conduction, and some of these also are active for the catalytic dissociation and reduction of oxygen. These so-called Mixed Ionic Electronic Conductors (“MIEC”) are often employed in cathode designs of solid oxide electrochemical cells. Such materials include, Samarium-doped Strontium Cobaltite (“SSC”), Lanthanum Strontium Ferrite (LSF), Lanthanum Strontium Cobalt Ferrite (“LSCF”).
To carry out the task of conducting electrons into the cathode and to a location where wires or other conductors may be attached for connection to external circuits, a current collector, composed of a porous layer of materials such as gold, silver, silver-palladium alloy, platinum, stainless steel, ferritic steel, In2O3 is employed.
It is clear from the above descriptions that various of these materials may be employed in multiple roles in a cell, with the actual material selection in any particular case often being based on economics, the compatibility of the physical and/or chemical properties of a combination of materials, simplicity of fabrication, etc.
Oxide-ion conducting electrochemical cells rely upon a gas-impermeable electrolyte component which conducts oxide (O2−) ions but which does not conduct electrons (i.e. the electronic conductivity is negligible). The oxide ions originate in the form of oxygen molecules in a gas phase, e.g. air or oxygen gas and must be converted to oxide ions and then introduced into the electrolyte. This is the function of the electron-conducting cathode electrocatalyst, which is in intimate contact with the electrolyte, and, more specifically, the function of the three-phase boundary (TPB) between the cathode, electrolyte, and gas phases. Since the cathode must allow the gas phase to penetrate to the cathode-electrolyte interface, the cathode is generally porous.
Present at the TPB are all of the species required for the production of oxide ions and their introduction into the electrolyte: molecular oxygen in the gas phase, electrons in the cathode, and oxide ion vacancies in the electrolyte. If the cathode material is an electronic conductor only, i.e. it does not conduct oxide ions, then the TPB comprises one or more one-dimensional line(s). However, if the cathode material is a mixed-electronic-ionic conductor (MIEC) then the TPB can be extended to the two-dimensional internal surface area of the porous cathode or, at least, the small portion of that area that is in close proximity to the electrolyte if the ionic conductivity of the MIEC is much lower that that of the electrolyte. The cathode may also consist of a combination of electron conducting, ion conducting, and MIEC materials.
In addition to the presence of the requisite species, to produce oxide ions and introduce them into the electrolyte at a useful rate, it is necessary that an electrocatalyst be present that mediates both the dissociation of molecular oxygen and the reduction of the resulting oxygen atoms to oxide ions. Either the entire cathode may consist of a porous structure of electrocatalyst, or the electrocatalyst may be present only in a portion of the cathode, e.g. that portion that is in close proximity to the electrolyte.
In general, the cathode structure also includes a current collector component, which is an outer layer composed of a porous material with high electronic conductivity and which is in electrical contact with at least one of the electron-conducting materials within the cathode structure. Wires are then generally bonded to the current collector in order to provide a high-conductance path for electron flow from the external circuit.
It is well known in the art that, currently, the rate-limiting steps in solid oxide electrochemical cell operation are those that take place at and near the cathode-electrolyte interface, i.e. the TPB. The reaction rate at the TPB depends on the total length or area of the TPB as well as the specific activity of the combination of materials and materials properties that compose the TPB. Therefore it is important to enhance both the extent of the TPB and its specific activity in order to produce higher-performance cells. Also, given that the reactions mentioned above and the mobility of oxide ions are thermally activated processes, such enhancement may also serve to lower the temperature at which adequate performance may be achieved. Lowering the operating temperature of the cell provides significant technological and economic advantages given the increased number of associated component materials that may be employed at lower temperatures as well as the increased conductivity of the electronic conductors that are used as current collectors.
Many of the materials that are used as electrocatalysts, such as LSM, SSC, etc., have low electronic conductivity relative to metals, and a porous layer of such materials possesses even lower conductivity. In conventional cathode structures, the cathode is relatively thick (˜25-50 μm) and causes undesirable resistive power losses (higher total cell resistance). Reduction of the thickness of the high-resistance portion of the cathode structure can therefore significantly increase the efficiency of the cell.
Known methods used to increase the spatial extent of the TPB include enhancing the surface area of the dense electrolyte. This may be done by roughening the surface by sintering electrolyte particles onto the surface or sintering a thick porous coating of electrolyte material onto the surface, thus increasing the total area of the electrolyte available to form the TPB. These methods may also increase the adhesion of the electrocatalyst as well as the current collector layer. However roughening the surface of the electrolyte results in only a minor increase in the surface area, and sintering a thick porous layer to the electrolyte surface requires using pore-formers, such as graphite particles, and generally results in relatively low porosity, high tortuosity, and many grain boundaries within the structure, all of which reduce the efficiency of the cell. In particular, it is known that a porous ion conductor which has a thickness large relative to the grain size has a decreased effective ion conductivity, indicating again that a reduction of the thickness of the cathode structure would be advantageous.